MRI Web Clinic — June 2008

Apophyseal Avulsion Injury of the Pelvis
Ingrid Kjellin, M.D.

Clinical history: A 15 year-old male soccer player presents with left hip pain and weakness for four weeks without known injury. He has severe pain over the left anterior superior iliac spine (ASIS) after activity. (1a) An axial fat-suppressed T2-weighted images of the upper pelvis is provided. What are the findings” What is the diagnosis?

1a
Figure 1:

(1a) An axial fat-suppressed T2-weighted images of the upper pelvis

Findings

2a
Figure 2:

(2a) A mildly displaced fracture (arrow) is seen through the physis of the left anterior superior iliac spine (ASIS). The edema in the fractured apophysis, iliac wing and adjacent soft tissues is relatively mild, which is indicative of a subacute injury. Hyperintense signal in the contralateral ASIS (arrowhead) represents growth cartilage in this skeletally immature individual.

3a
Figure 3:

(3a) A frontal radiograph of the left hip in the same patient shows subtle cortical irregularity of the ASIS (arrow).

Diagnosis

Apophyseal avulsion injury of the anterior superior iliac spine (ASIS).

The ASIS apophysis starts to ossify between the ages of 13 and 15 years and fuses with the ilium between the ages of 21 and 25 years. The ASIS is the attachment site for the sartorius and some fibers of the tensor fascia latae. Avulsion of the ASIS occurs from a strong sudden pull of the sartorius with the hip in extension and the knee in flexion, most commonly in sprinters, hurdlers and other running athletes.

Introduction

Apophyseal avulsion injuries to the hip and pelvis account for 10 to 24 % of athletic injuries in children.1 The most commonly implicated sports and physical activities are soccer, running and ballet dancing.1 Similar injuries also occur in football, baseball and track.2 In skeletally immature individuals, injuries caused by excessive tensile force on the muscle-tendon-bone unit tend to result in apophyseal avulsion fractures, because the weakest biomechanical point is the growth plate. When an apophyseal avulsion is nondisplaced or when the apophysis is not yet ossified, radiographs may be interpreted as negative. MRI may be extremely helpful for diagnosis and clinical management in such cases.

Discussion

In the adolescent competitive athlete the pelvis is a common location of apophyseal avulsion injury. In a study of 203 avulsion fractures seen on radiographs,3 the most commonly affected apophyses were: (1) the ischial tuberosity (the origin of the hamstrings); (2) the anterior inferior iliac spine, AIIS (the origin of the straight head of the rectus femoris); (3) the anterior superior iliac spine, ASIS (the origin of the sartorius and some fibers of the tensor fascia lata); and (4) a portion of the pubic symphysis (the origin of the adductor brevis and longus as well as the gracilis)(4a).

4a
Figure 4:

(4a) A 3-dimensional model of the pelvis illustrates the most common sites of apophyseal avulsion injury in adolescents, depicted in blue. Less common sites of apophyseal avulsion are indicated in red. Illustration courtesy of Michael E. Stadnick, M.D.

The mechanism of injury in apophyseal avulsions is sudden forceful concentric or eccentric muscle contraction during running, jumping or kicking a ball, which results in traction on the unfused apophysis. Extreme passive stretching and chronic repetitive microtrauma have also been implicated in the development of apophyseal avulsion. When a strain-type injury occurs it is the patient’s age that tends to determine where the disruption occurs in the chain of bone, tendon and muscle. In a young adult, the failure usually involves the myotendinous junction. In an older adult, the failure tends to target the tendon, which is often weakened by tendinosis. The weak link in the muscle-tendon-bone chain in children and adolescents is the physis (growth plate), especially at times of growth acceleration.4,5

Clinically, patients present with pain and weakness, swelling after activity, point tenderness, discoloration and altered gait. In the subacute or chronic setting, especially when no history of trauma is provided, the initial working diagnosis is often neoplasm or infection.

Radiographic findings are often absent or subtle, but in cases of excessive callus, may mimic an osteosarcoma, osteochondroma or an infectious process.2 Knowledge of the tendon attachments to the pelvis is essential in arriving at the correct diagnosis. MR imaging of apophyseal avulsions allows direct visualization of the tendon attached to the avulsed apophysis. The involved tendon may have a lax configuration due to retraction. Variable degrees of edema in the bone and surrounding fat and muscle are present on water sensitive images. Avulsed cortical bone is hypointense and may be indistinguishable from adjacent tendons or ligaments. CT provides better delineation of bone detail from an avulsed apophysis as compared to MRI, and should be considered where the diagnosis is in doubt and when surgery is contemplated.

Additional examples of apophyseal avulsion fractures of the pelvis are provided below.

5a
5b
Figure 5:

Avulsion of the right ischial tuberosity in a teenage athlete. (5a) T1-weighted coronal and (5b) fat-suppressed proton density-weighted sagittal images demonstrate an apophyseal avulsion with marrow edema in the apophysis and adjacent ischium. Abnormal widening of the apophysis is readily apparent (arrows). The semimembranosus tendon (arrowhead) attaching to the apophysis is intact.

The ischial apophysis starts to ossify between the ages of 14 and 16 years and usually fuses with the ischium between the ages of 18 and 21 years. The hamstring muscles originate from the ischial tuberosity, and the mechanism of injury includes a sudden contraction of the hamstrings muscles with the knee extended and the hip flexed. Hamstrings injuries have been discussed in detail in a previous Web Clinic.

6a
Figure 6:

14 year-old male with avulsion of the left AIIS. A (6a) T2-weighted coronal image reveals the avulsion of the apophysis at the direct head of the rectus femoris (arrow) with associated marrow and soft tissue edema(arrowheads). Note the normal AIIS and rectus femoris tendon on the right (blue arrow).

7a
Figure 7:

A (7a) sagittal proton density image with fat saturation of the left hip in the same patient depicts the avulsion injury with complete disruption of the direct head (arrow) and partial tearing of the indirect/reflected (arrowhead) head of the rectus femoris.

AIIS avulsion, also known as sprinter’s fracture, occurs from overpull of the straight head of the rectus femoris when the hip is hyperextended and the knee is in flexion. The AIIS apophysis starts to ossify between the ages of 13 to 14 years and fuses with the ilium between the ages of 16 and 18 years.

8a
Figure 8:

A 15 year-old girl with bilateral iliac crest avulsion on a (8a) coronal STIR image. Marrow edema and cortical irregularity (arrows) is evident at both iliac crests.

Iliac crest apophyseal avulsion injury is relatively rare and occurs with pulling of one or more of the muscles that have their attachment to the iliac crest (the external and internal abdominal oblique muscles, transverse abdominis muscle, gluteus medius muscle, and tensor fascia latae). Ossification of the iliac crest apophysis starts along the anterolateral aspect of the iliac crest between the ages of 13-15 years, and continues posteromedially. The fusion of the ossified apophysis usually starts at the age of 15 years and continues to the age of 25 years.

9a
Figure 9:

Iliac crest apophyseal avulsion on the left (arrow) shown on a (9a) plain radiograph in a different patient. Bone resorption is present in the adjacent anterior superior iliac spine (arrowhead), consistent with ASIS avulsion. In a small minority of cases, avulsion injuries are multiple at the time of presentation.

10a
Figure 10:

A (10a) STIR coronal image of the symphysis pubis in a 16 year-old male soccer player, who was initially thought to have osteomyelitis. Diffuse signal abnormality in the right superior pubic ramus (arrow) is present.

11a
Figure 11:

(11a) Inferior contour irregularity and marrow signal abnormality are demonstrated in the same patient in the right parasymphyseal region (arrow) at the origin of the adductor brevis, consistent with apophyseal avulsion injury.

Treatment and Prognosis

Nondisplaced apophyseal avulsions of the pelvis are usually treated with nonsteroidal anti-inflammatory agents, activity modification, and rehabilitation.6 After resolution of clinical symptoms, which usually takes 4-6 weeks, the patient can gradually return to athletic activity. Surgery is considered for recent apophyseal avulsion fractures displaced more than 2 cm. In cases of malunited or hypertrophied fragments, surgery may also be beneficial.7 Permanent disability and limitation of sporting activity have been reported.8

Conclusion

Apophyseal avulsion injuries of the pelvis have occurred with increasing frequency over the past several decades because of the increased participation of children and adolescents in competitive athletic activities. The most commonly avulsed apophysis is the ischial tuberosity, followed by the AIIS and the ASIS. Radiographs may be interpreted as negative when an apophyseal avulsion is nondisplaced or when the apophysis is unossified. Alternatively, exuberant callus formation may mimic neoplasm or osteomyelitis, confusing the clinical picture. Knowledge of anatomy allows the experienced interpreter of MRI to correctly identify the characteristic appearance and location of apophyseal avulsion injuries. MRI allows evaluation of the extent of injury and osseous displacement, both of which are important factors in treatment decisions. Associated muscle and tendon strain injuries are also well demonstrated.

References

1 Boyd KT, Peirce NS, Batt ME. Common hip injuries in sports. Sports Med 1997;24(4):273-288.

2 Stevens MA, El-Khoury GY, Kathol MH et al. Imaging features of avulsion injuries. Radiographics 1999;19(3):655-672.

3 Rossi F, Dragoni S. Acute avulsion fractures of the pelvis in adolescent competitive athletes: prevalence, location and sports distribution of 203 cases collected. Skeletal Radiol 2001;30(3):127-31.

4 Combs J. Hip and pelvis avulsion injuries in adolescents. Phys Sports Med 1994; 22:41-49.

5 Micheli LJ, Fehlandt AF Jr. Overuse injuries to tendons and apophyses in children and adolescents. Clin Sports Med 1992;11:713-726.

6 Diehl JJ, Best TM, Kaeding CC. Classification and return-to-play considerations for stress fractures. Clin Sports Med 2006;25:17-28.

7 Kujala UM, Orava S. ischial apophysis injuries in athletes. Sports Med 1993;16(4):290-294.

8 Manthravadi S, Carty H. Avulsion fracture of the pelvis in children: a report of 32 fractures and their outcome. Skeletal Radiol 1994; 23(2) 85-90.

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